Question

我无法理解此错误消息，不太确定我接下来要调查的内容。

我有以下导入：

{-# LANGUAGE OverloadedStrings #-}
module Main where

import Web.Scotty

import Control.Applicative
import Control.Monad.IO.Class
import Database.SQLite.Simple
import Database.SQLite.Simple.FromRow
import qualified Data.Text.Lazy as L

导致错误的代码：

routes :: ScottyM ()
routes = do
  post "/create" $ do
    f <- param ("fa" :: L.Text)
    file "create.html"

错误：

    • Ambiguous type variable ‘a0’ arising from a use of ‘param’
  prevents the constraint ‘(Parsable a0)’ from being solved.
  Probable fix: use a type annotation to specify what ‘a0’ should be.
  These potential instances exist:
    instance Parsable Integer
      -- Defined in ‘scotty-0.11.0:Web.Scotty.Action’
    instance Parsable L.Text
      -- Defined in ‘scotty-0.11.0:Web.Scotty.Action’
    instance Parsable ()
      -- Defined in ‘scotty-0.11.0:Web.Scotty.Action’
    ...plus 7 others
    ...plus 12 instances involving out-of-scope types
    (use -fprint-potential-instances to see them all)
• In a stmt of a 'do' block: f <- param ("f" :: L.Text)
  In the second argument of ‘($)’, namely
    ‘do { f <- param ("f" :: L.Text);
          file "create.html" }’
  In a stmt of a 'do' block:
    post "/create"
    $ do { f <- param ("f" :: L.Text);
           file "create.html" }

Answer 1

param的类型为Parseable a => Text -> ActionM a，这意味着f的类型为a。但是，您永远不会对f执行任何操作，因此无法推断a应该是什么类型。假设您不希望仅在使用f之前对该行进行注释，则需要提供一个显式类型：

routes :: ScottyM ()
routes = do
  post "/create" $ do
    f <- param ("fa" :: L.Text) :: ActionM L.Text
    file "create.html"

您可以选择具有Parseable个实例的任何类型，但L.Text似乎应的类型。实际使用f后，您可以删除显式注释。

使用'param'产生的模糊类型变量'a0'可以防止约束'（Parsable a0）'被解决

1 个答案: